PRINTING PLATE MATERIAL AND PROCESS FOR PRODUCING THE SAME

Abstract: A printing plate material which can form uneven patterns on its surface even with a low laser energy and enables printing while preventing scumming of a print is produced by forming on a support a photosensitive layer containing nitrocellulose, a black coloring agent, such as carbon black, and an organic binder and having either (1) a surface roughness R max of 2.0 µm or less or (2) wettability index of 36 dyn/cm or less. The nitrocellulose has a nitrogen content of 11-12.5 % and a solution viscosity of about 1-1/8 sec. The organic binder contains a polyester resin such as a solvent-soluble, non crystalline and linear oil-free aromatic polyester. The nitrocellulose and the coloring agent are used in amounts of, respectively, 5-300 parts by weight and 0.5-50 parts by weight per 100 parts by weight of the organic binder. The photosensitive layer may contain a cross-linking agent such as a melamine resin and a water repellent such as a silicone or fluorine compound. An example of the usable support is a polyethylene terephthalate film having a mean density of less than 1.39.

Description: This invention relates to a printing plate which can be sculptured with laser light for use in gravure printing, among other uses, and to a method for its production. A laser-sculptured printing plate is known which comprises a mixture of polyethylene and carbon black as molded in the form of a sheet. However, this printing plate requires high-energy laser radiation for creating a concave-convex (three-dimensional) pattern on its surface. For this reason, any printing plate on which a three-dimensional pattern can be produced with a low-energy laser radiation has not been commercially implemented yet. Meanwhile, a recording material supplemented with nitrocellulose for enhanced sensitivity is known but this material is deficient in the strength required of any printing plate. By way of example, Japanese Patent Publication No. 51-35144 discloses an image forming technology which comprises coating a support with a photosensitive composition containing nitrocellulose and carbon black and irradiating the coated support with laser light from behind the support to form an image. Described in this prior art literature is an embodiment in which an ink is transferred to the photosensitive layer for printing. Moreover, Japanese Patent Application Laid-open No. 5-8367 discloses a gravure printing plate having a photosensitive layer containing nitrocellulose and carbon black. However, these photosensitive layers are invariably of low strength so that the final materials are not practically useful as printing plates. The strength of a printing plate is not only dependent on the strength of the photosensitive layer but also on the strength of the support. For example, when a crystalline polyester film, for example a biaxially-oriented polyethylene terephthalate film, is used for the support, its strength varies according to processing conditions such as coating with a photo-sensitive composition. Thus, the inherent performance characteristics of the crystalline polyester are not fully expressed so that the printing plate may not be imparted with sufficiently high strength. Furthermore, any printing plate is required to have a high degree of surface smoothness in order to preclude background staining of the print due to residues of the printing ink on the plate surface. However, the surface smoothness of the printing plate manufactured by using a photosensitive composition containing nitrocellulose and carbon black is not so high. The rough plate surface is not conducive to soil-free prints. Particularly when the printing ink is a water-based ink which is easy to handle and high in safety and hygienics, it is difficult to scrape off the excess ink from the printing plate with a blade even if the plate has a smooth surface, probably because the ink used has a higher surface tension than the gravure printing ink containing an organic solvent. The failure to clean the plate surface thoroughly with a blade leads to ink residues on the plate and, hence, stained prints. Japanese Patent Publication No. 51-6569 discloses an image-forming printing plate having a photosensitive layer containing nitrocellulose as a self-oxidative binder, carbon black as a particulate pigment capable of absorbing laser energy, and melamine resin as a curing agent (crosslinking agent). This literature describes examples in which an alkyd resin, a methyl methacrylate resin, a butyral resin, an epoxy resin or a novolac resin is further used. However, the photosensitive material described in the above literature is designed for use in the process for producing a lithographic or planographic printing plate which comprises contacting the photosensitive coating layer coated on a support with the polyvinyl alcohol coating layer coated on an aluminum plate and irradiating them with laser light from behind the support to thereby transfer an image corresponding to the irradiated area to the aluminum plate. Therefore, it is not a printing plate belonging to the category in which a printing ink is directly transferred on a photosensitive layer for printing. Moreover, the sensitivity and film strength of the photosensitive layer formed with the photosensitive material are not sufficiently high so that the product can hardly be used, as it is, as a printing plate. Therefore, it is an object of the present invention to provide a printing plate on which a concave-convex (three-dimensional) pattern can be formed even at a low laser energy output and which has a high film strength and a method of producing the printing plate. It is a further object of the present invention to provide a printing plate havig a high sensitivity relative to laser light and useful to form a concave-convex pattern with high accuracy and a method of producing the printing plate. It is a still further object of the present invention to provide a printing plate which can be used in a printing process comprising transferring a printing ink directly on the surface of its layer having a concave-convex pattern produced with laser light and has excellent strength, durability and wear resistance (printing pressure resistance) in association with printing and a method of producing the printing plate. It is still another object of the present invention to provide a printing plate having a high degree of surface smoothness and inhibiting a background staining of the print and a method of producing the printing plate. A further object of the present invention is to provide a printing plate which can be cleaned by scraping off an excess of printing ink from its surface by means of a blade even when a water-based printing ink is employed and, therefore, provides for a minimum background staining of the print and a method of producing the printing plate. The inventors of the present invention did intensive research to accomplish the above-mentioned objects and found that by forming a photosensitive layer comprising nitrocellulose, a black coloring agent and a specific organic binder, a printing plate having a photosensitive layer which is highly thermodegradable on exposure to laser radiation and has a high film strength can be obtained and that the surface smoothness and surface tension of the photosensitive layer affect remarkably on printability. The present invention has been developed on the basis of the above findings. The present invention relates to the use of a material for a printing plate, wherein said material comprises a support having provided thereon a photosensitive layer containing nitrocellulose, a black coloring agent and, as an organic binder, a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents, said photosensitive layer having (1) a surface roughness of not greater than Rmax. 2.0 µm or (2) a surface tension equivalent to a wettability index of not greater than 36 mN/m. The printing plate of the present invention comprises a support having provided thereon a photosensitive layer containing nitrocellulose, a black coloring agent and a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents, said photosensitive layer having (1) a surface roughness of not greater than Rmax. 2.0 µm or (2) a surface tension equivalent to a wettability index of not greater than 36 mN/m. As the nitrocellulose, a variety of nitrocelluloses which can be thermally decomposed with laser light, for example a nitrocellulose with a nitrogen content of 10 to 14%, a degree of polymerization within the range of 10 to 1500, and a solution viscosity according to Japanese Industrial Standards (JIS) K6703 of 20 to 1/10 second, may be used. The black coloring agent includes a variety of substances capable of absorbing laser light, for example, carbon black. For improved strength, the photosensitive layer may contain a crosslinking agent. Furthermore, the photosensitive layer may contain a water repellent for controlling the surface wettability index. As the support, a flexible transparent support such as a biaxially oriented polyethylene terephthalate film may be used. Since the support affects on the strength of the printing plate, the density of the polyethylene terephthalate film support is preferably less than 1.39. The above printing plate can be used for e.g. gravure printing or flexo printing, and, as the printing ink, a water-based printing ink is usually employed. The above printing plate may be manufactured by coating a support, for example a biaxially stretched or oriented polyethylene terephthalate film having a density of less than 1.39, with a coating composition comprising nitrocellulose, a black coloring agent and a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents to provide a photosensitive layer which can be sculptured with laser light. The coating composition mentioned above may further contain a crosslinking agent and/or a water repellent. As used throughout this specification, the term "film" includes a sheet unless otherwise specified. The species of nitrocellulose to be incorporated in the photosensitive layer is not particularly restricted only if it is thermally degradable. The nitrogen content of the nitrocellulose is generally 10 to 14%, preferably 11 to 12.5%, and more preferably 11.5 to 12.2%. The degree of polymerization of the nitrocellulose can also be liberally selected within a broad range of, for example, 10 to 1,500. The preferred degree of polymerization of nitrocellulose may, for example, be 10 to 900 and particularly 15 to 150. The preferred nitrocellulose includes a nitrocellulose with a solution viscosity of 20 to 1/10 second, preferably 10 to 1/8 second, as determined in accordance with JIS K6703 "Industrial Cellulose" (the viscosity denomination of Hercules Powder Company). The practically used nitrocellulose has a solution viscosity of 5 to 1/8 second, particularly 1 to 1/8 second. If necessary, two or more species of nitrocellulose can be used in combination. The amount of nitrocellulose may be selected from the range not adversely affecting the sensitivity of the photosensitive layer and may be 5 to 300 parts by weight, preferably 20 to 200 parts by weight, and more preferably 30 to 150 parts by weight relative to 100 parts by weight of the organic binder. The nitrocellulose may be practically used in an amount of 5 to 100 parts by weight relative to 100 parts by weight of the organic binder. The black coloring agent includes a variety of absorbents which absorb laser energy with high efficiency, for example black dyes and carbon black. The preferred species of the black coloring agent includes carbon black and insofar as a high dispersion stability can be imparted to the composition, any of the species defined in ASTM and/or indicated for whatever uses (e.g.colors, rubber, dry batteries) can be employed. For example, the carbon black includes furnace black, thermal black, channel black, lamp black and acetylene black. The black coloring agent such as carbon black can be used in the form of color chips or a color paste, which can be prepared by dispersing it in nitrocellulose and/or an organic binder beforehand, in which case the dispersion thereof is facilitated. Such chips and paste are readily available from commercial sources. The amount of the black coloring agent may be selected from the range not adversely affecting the sensitivity of the photosensitive layer and may be, for example, 0.5 to 50 parts by weight, preferably 5 to 40 parts by weight, and more preferably 10 to 35 parts by weight relative to 100 parts by weight of the organic binder. The black coloring agent may be used practically in an amount of 10 to 40 parts by weight relative to 100 parts by weight of the organic binder. In addition to the above linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents, the photosensitive layer may contain other organic binders unless same do not adversely affect the laser- light sensitivity and printing pressure resistance. Such additional binders include thermosettable resins such as a phenolic resin, an epoxy resin or a vinyl ester resin, thermoplastic resins such as a polyvinyl chloride, an acrylic resin, a styrenic resin, a polycarbonate, a polyester resin not being a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents, or a polyamide, and other resins such as naturally-occurring resins. These organic binders may be used singly or in combination. Preferred additional organic binders include thermoplastic resins, particularly those resins which can be cured with a crosslinking agent. The preferred additional binders include polyester resins which can be provided with high sensitivity to laser light and are highly solvent-and wear-resistant , particularly aromatic polyesters. When the nitrocellulose, black coloring agent and polyester resin are used in combination, a concave-convex pattern (three-dimensional pattern) can be formed with high precision even at a low laser energy output and, in addition, the film strength of the photosensitive layer can be increased. Furthermore, a high printing press resistance can be expected even when a printing ink is directly transferred to the three-dimentionally patterned photosensitive layer. As the additional polyester resin, a variety of polyesters obtainable by condensing a polycarboxylic acid or an acid anhydride or lower alcohol ester thereof with a polyhydric alcohol can be employed. Moreover, hydroxycarboxylic acids may also be used in the condensation reaction. The polycarboxylic acid component includes, for example, aromatic carboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, a methyl-substituted terephthalic acid, trimellitic acid or pyromellitic acid, and the corresponding anhydrides; saturated or unsaturated aliphatic carboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, maleic acid or fumaric acid, and the corresponding anhydrides. The practically preferred polycarboxylic acid component includes aromatic dicarboxylic acids such as phthalic acid, isophthalic acid or terephthalic acid, and the corresponding anhydrides, and saturated aliphatic dicarboxylic acids such as adipic acid, azelaic acid or sebacic acid. The hydroxycarboxylic acid includes, for example, β-hydroxypropionic acid and β-hydroxybutyric acid. As the polyhydric alcohol component, there may be mentioned, for example, aliphatic dihydric alcohols such as ethylene glycol, propylene glycol, trimethylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol or neopentyl glycol; polyoxyalkylene glycols such as diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol or tetrapropylene glycol; aliphatic polyhydric alcohols such as glycerin, trimethylolpropane, trimethylolethane or pentaerythritol; alicyclic polyhydric alcohols such as a cyclohexanediol or hydrogenated bisphenol A; and aromatic polyhydric alcohols such as bisphenol A-alkylene oxide adducts, e.g. 2,2-bis(4-dihydroxyethylphenyl)propane and 2,2-bis(4-dihydroxypropylphenyl)propane. The polyester resin may have hydroxyl and/or carboxyl groups. The molecular weight of polyester resin may for example be 1000 to 100000, and preferably 2000 to 50000. The polyester resin used in the present invention is a linear polyester which is soluble in organic solvents and non-crystalline. Such a polyester resin includes aromatic ring-containing polyesters such as oil-free aromatic polyesters having high resistance to solvents and wear. The aromatic ring of the polyester is usually derived from an aromatic polycarboxylic acid such as terephthalic acid, phthalic acid or isophthalic acid. Such polyester resins include modified polyesters (modified polyethylene terephthalate) prepared by partial replacement of the ethylene glycol and/or terephthalic acid unit of polyethylene terephthalate with a different diol (e.g. triethylene glycol, cyclohexanedimethanol) and/or dicarboxylic acid (e.g. phthalic acid, isophthalic acid, adipic acid). For inhibition of background staining of prints, the above-mentioned photosensitive layer preferably contains a water repellent. As examples of the water repellent, there may be mentioned silicone compounds represented by organosiloxanes (e.g. silicone oils such as polydimethylsiloxane, polydiphenylsiloxane, polymethylphenylsiloxane; modified silicones such as a polyether-modified dimethylsiloxane, an amino-modified polydimethylsiloxane; silicone gum (gum-like silicone), silicone rubber, silane coupling agents containing a reactive group such as amino, glycidyl, methoxy), fluorine compounds represented by fluorocarbons (e.g. low molecular weight compounds such as fluorocarbon, benzotrifluoride, perfluorobenzene, perfluoromethyldecaline, a homo- or co-polymer of chlorotrifluoroethylene which has a lower degree of polymerization; and polymers such as fluororesin, a homo- or co-polymer of fluorine-containing (meth)acrylic monomer), and those copolymers and graft polymers which contain an organosiloxane or fluorocarbon unit. The copolymers and graft copolymers mentioned just above include a functional comb-shaped graft copolymer which is obtainable by copolymerizing a macromonomer with functional monomers (for example unsaturated compounds such as fluorine- or silicon-containing acrylic monomers) and these polymers are commercially available from Toagosei Chemical Industry Co., Ltd., Japan as "Aron G Series" fluorine-containing graft polymers and silicone graft polymers. These water repellents are generally nonvolatile at room temperature and either liquid, viscous or resinous. These water repellents can be obtained from commercial sources. Thus, polydimethylsiloxane, among silicone water repellents, is available from Toshiba Silicone Co., Ltd. under the tradename of TSF-451-100 and TSF-451-1M, and silicone gums can be obtained from Shin-Etsu Silicone Co., Ltd. A silicone comb-shaped polymer water repellent is available from Toagosei Chemical Industry Co., Ltd. under the tradename of Aron G GS-30. A fluorine-containing comb-shaped polymer water repellent is also available from Toagosei Chemical Industry Co., Ltd. as the tradename of Aron G GF-300. A fluorine- and silicon-containing comb-shaped polymer water repellent may be purchased from Toagosei Chemical Industry Co., Ltd. under the tradename of Aron G GF-150. The amount of the water repellent relative to the photosensitive layer may be selected from the range providing a wettability index (surface tension) of not more than 36 mN/m (dyn/cm), preferably 30 to 36 mN/m (dyn/cm) and more preferably 32 to 36 mN/m (dyn/cm)to the surface of the printing plate. The proportion of the water repellent relative to 100 parts by weight of the photosensitive layer in terms of the nonvolatile content of the photosensitive coating composition is generally 0.1 to 5 parts by weight, preferably 0.15 to 4 parts by weight, and more preferably 0.2 to 3 parts by weight. Practically, the water repellent is used in a proportion of 0.2 to 2 parts by weight relative to 100 parts by weight of the photosensitive layer. If the surface wettability index of the printing plate exceeds mN/m (36 dyn/cm), background stain of the print occurs to detract from the degree of definition or visibility of the print. Particularly when a water-based printing ink such as a water-based gravure printing ink or a water-based flexo printing ink is employed, such staining or soling of prints is prominent. When a printing plate with a surface tension within the above range is employed, residues of the printing ink on the plate can be efficiently scraped off with a doctor blade so that background stain of the print due to the residual ink can be prevented. Thus, a print of sharp image definition can be produced. Where necessary, a crosslinking agent is incorporated in the photosensitive layer. The crosslinking agent includes, for example, amino resins such as a melamine resin, a benzoguanamine resin or a urea resin, and organic peroxides such as benzoyl peroxide, di-t-butyl peroxide or methyl ethyl ketone peroxide. As specific crosslinking agents that can be obtained from commercial sources, the tradename "Super-Beckamine L-105-60" (Dainippon Ink and Chemicals, Inc.) and the tradename "Cymel 325" and "Cymel 303" (Mitsui-Cyanamid, Co., Ltd.) can be mentioned. The proportion of the crosslinking agent may for example be 0.1 to 40 parts by weight and preferably 1 to 30 parts by weight relative to 100 parts by weight of the polyester resin. If required, the photosensitive layer may further contain a variety of additives, for example, plasticizers, antioxidants, ultraviolet absorbers or other aging inhibitors, cure accelerators, acid catalysts (e.g. the tradename "BYK catalyst 450"; BYK Co.), flame retardants, fillers, coloring agents or levelling agents. The shape of the support is not restricted only if it is a printable form and may for example be drum-like, film-like or sheet-like. For irradiation with laser beams through the support, a laser light- transparent support is used as the support. Since the printing plate is put to use as wrapped around the drum (printing drum) of a printing machine, the support is usually supplied in the form of a film or sheet. Although the material of the support is not, particularly limited, a transparent plastic film is usually employed when a support of film or sheet form is chosen. For the plastic film, a variety of films including polypropylene, polyester and polyamide films can be employed and these films are generally used in stretched or oriented form, particularly in the form of a biaxially stretched or oriented film. In view of their toughness and dimensional accuracy or stability of pits, which are formed by laser beam irradiation, of the photosensitive layer in the printing process, polyalkylene terephthalates such as polyethylene terephthalate or polybutylene terephthalate, and especially polyethylene terephthalate are preferred. With regard to polyethylene terephthalate films, films of various grades can be employed but a film of the printing plate grade is preferred from the standpoint of e.g. accuracy of thickness or surface smoothness. In the case of a polyethylene terephthalate film, its average density has an important role on impact resistance. Thus, when the average density of a polyethylene terephthalate film is less than 1.39 and preferably 1.375 to 1.387, the impact resistance of the film is so high that the strength, durability and printing press resistance of the printing plate are enhanced. The impact resistance of the film is not affected by heat treatment but when its average density is higher than 1.39, the impact resistance of the polyethylene terephthalate film is decreased probably because the degree of crystallization is increased as the synergistic effect of the coating composition, the printing ink and the heat applied in the coating process and/or in the printing process. While the degree of crystallization varies from the surface to the inner part of the film, the impact resistance of the film is sharply decreased when its average density exceeds 1.39. The thickness of the support film can be suitably selected within the range providing for a high mechanical strength and, sufficient flexibility of the printing plate and may, for example, be 50 to 500 µm, preferably 75 to 300 µm. It should be noted that when such a thicker film is employed, the impact resistance may decrease considerably upon application of the coating composition. To preclude background staining of the print due to residues of the printing ink on the printing plate, the photosensitive layer formed on the support should preferably present a smooth surface. The surface roughness Rmax. of the photosensitive layer is generally not greater than 2.0 µm, preferably not greater than 1.5 µm (e.g. 0.1 to 1.2 µm), and more preferably not greater than 1.0 µm (e.g. 0.1 to 0.8 µm). When Rmax. is not greater than 1.0 µm, background staining of the print can be certainly precluded. With a printing plate having a highly surface smooth photosensitive layer, residues of the printing ink on the printing plate can be efficiently scraped of if so that background staining of the print due to the residual ink can be prevented. For this reason, a print of good image definition can be obtained even with a water-based printing ink. The surface of the more preferred photosensitive layer has a surface roughness Rmax. of not greater than 2.0 µm (preferably not greater than 1.5 µm) and a wettability index of not greater than 36 mN/m (dyn/cm) (preferably 32 to 36 mN/m (dyn/cm)). The thickness of the photosensitive layer formed on the support can be selected within the range not adversely affecting the durability of the photosensitive layer and may, for example, be 1 to 100 µm, preferably 10 to 50 µm, and practically 15 to 30 µm. If necessary, an adhesive layer or a under coating layer may be interposed between the photosensitive layer and the support, or the support may be surface-treated. Further, an antistatic layer may be formed on the reverse or opposite side of the support with respect to the photosensitive layer. The above-mentioned photosensitive layer may be formed by coating such a support with a coating composition comprising nitrocellulose, a black coloring agent and the organic binder. The coating composition may be prepared by mix-dispersing the nitrocellulose, the black coloring agent and the organic binder in an organic solvent using the conventional mixer or dispersing machine. To achieve an enhanced surface smoothness of the photosensitive layer, the black coloring agent (carbon black) is preferably highly dispersed. The dispersibility of the coloring agent can be controlled by adjusting the dispersing mixer shearing force, system viscosity, the proportion of nitrocellulose and/or organic binder relative to the coloring agent, and dispersing time. Within the range not adversely affecting the printing performance and printing press resistance, a surfactant or other dispersing agent may be incorporated to achieve an increased dispersibility of the coloring agent. The organic solvent mentioned hereinbefore includes, for example, alcohols such as ethanol or isopropyl alcohol; aliphatic hydrocarbons such as hexane or octane; alicyclic hydrocarbons such as cyclohexane; aromatic hydrocarbons such as benzene, toluene or xylene; halogenated hydrocarbons such as dichloromethane or dichloroethane; ketones such as acetone, methyl ethyl ketone or methyl isobutyl ketone; esters such as ethyl acetate or butyl acetate; ethers such as diethyl ether, tetrahydrofuran, cellosolve or carbitol; and various mixtures thereof. As the photosensitive layer formed in the above manner is irradiated with laser light image-wise, i.e. in accordance with a predetermined pattern, there is obtained a printing plate having a three-dimensional pattern (pits) in the photosensitive layer. Where the support is transparent, the laser beam may be projected from the support side to the photosensitive side but, in many instances, irradiation is performed from the photosensitive layer side. As the source of laser radiation, there may be used various types of lasers such as Ar lasers or He-Ne lasers, long-wavelength lasers whose emissions are readily absorbed by the black coloring agent, such as carbon black, to produce heat, such as a CO 2 laser, a YGA laser or a semiconductor laser can be employed with advantage. The preferred laser includes semiconductor lasers which are simple in structure. INDUSTRIAL APPLICABILITY The printing plate of the present invention can be used as a printing plate for e.g. gravure printing or flexo printing, and is suited for printing with a water-based printing ink, e.g. a water-based gravure printing ink or a water-based flexo printing inks. EXAMPLES The following examples describe this invention in further detail. Example 1 A paint shaker preloaded with steel beads (2 mm in diameter) was charged with 14.4 parts by weight of nitrocellulose RS 1/4 (Daicel Chemical Industries, Ltd., isopropyl alcohol-wetted, nonvolatile 70 % by weight), 3.7 parts by weight of carbon black (Mitsubishi Chemical Industries Ltd., MA100), 78.1 parts by weight of a polyester resin (Toyobo Co, Ltd., Vylon 20SS, nonvolatile 30 % by weight), 10.9 parts by weight of methyl ethyl ketone, 4.3 parts by weight of isopropyl alcohol, and 8.6 parts by weight of toluene were mixed and dispersed by shaking for 120 minutes. Using an applicator, the resulting coating composition was coated in a dry thickness of 20 µm on a biaxially oriented polyethylene terephthalate film (100 µm thick) and dried at 120°C for 3 minutes to provide a printing plate. The surface roughness of the printing plate was Rmax. 1.2 µm. Example 2 A coating composition was prepared by mix-dispersing 7.7 parts by weight of nitrocellulose RS 1/4 (Daicel Chemical Industries, Ltd., isopropyl alcohol-wetted, nonvolatile 70 % by weight), 24.6 parts by weight of a carbon black paste (Toyo Ink Manufacturing, Co., Ltd., Color Paste ANP-C-903 Black, carbon black content 12 % by weight, nitrocellulose content 18 % by weight), 43.3 parts by weight of a polyester resin (Toyobo Co., Ltd., Vylon 20SS, nonvolatile 30 % by weight), 5.4 parts by weight of a crosslinking agent (Dainippon Ink and Chemicals, Inc., melamine resin, Super-Beckamine L-105-60, nonvolatile 60 % by weight), 0.4 part by weight of acid catalyst (BYK Co., BYK Catalyst 450, nonvolatile about 40 % by weight), and 18.7 parts by weight of methyl ethyl ketone. Using the resulting coating composition, a printing plate was prepared in the same manner as in Example 1. The surface roughness of the plate was Rmax. 0.5 µm. Example 3 A coating composition was prepared by dispersing together 6.4 parts by weight of nitrocellulose RS 1/4 (Daicel Chemical Industries, Ltd., isopropyl alcohol-wetted, nonvolatile 70 % by weight), 29.9 parts by weight of carbon black-containing chips (Taisei Kako, Co., Ltd., TC Color Chip 5307, carbon black content 30 % by weight, nitrocellulose content 52 % by weight, plasticizer 18 % by weight), 97.2 parts by weight of a polyester resin (Toyobo Co., Ltd., Vylon 30SS, nonvolatile 30 % by weight), 13.5 parts by weight of a cross-linking agent (Dainippon Ink and Chemicals, Inc., melamine resin, Super-Beckamine L-105-60, nonvolatile 60 % by weight), 0.9 part by weight of an acid catalyst (BYK Co., BYK Catalyst 450, nonvolatile about 40 % by weight), and 101.6 parts by weight of methyl ethyl ketone. Using the coating composition thus obtained, a printing plate was obtained in the same manner as in Example 1. The surface roughness of the plate was Rmax. 0.8 µm. Comparative Example 1 A printing plate was obtained in the same manner as in Example 1 except that the shaking time for the preparation of a coating composition was 60 minutes. The surface roughness of this plate was 2.5 µm. Comparative Example 2 A printing plate was obtained by repeating the procedure of Example 1 except that the amount of carbon black used and the shaking time in the preparation of a coating composition was 20 parts by weight and 120 minutes, respectively. The surface roughness of the resulting plate was Rmax. 4.5 µm. The surface smoothness of the photosensitive layer, the background stain of prints, and the solvent-and water-resistance of the photosensitive layer were evaluated as follows. Surface smoothness: Evaluated in terms of Rmax, according to the following criteria. Excellent: Rmax. ≦ 1 µm Good : Rmax. = 1 - 2 µm Fair : Rmax. = 2 - 3 µm Poor : Rmax. > 3 µm Print background stain: The unpatterned printing plate was wrapped around the drum of a modified gravure printing machine and using a water-based printing ink and coated paper, 3000 impressions were made and the resulting prints were evaluated according to the following criteria. Excellent: No background stain Good : Slight background stain Poor : Overt background stain Solvent-resistance of photosensitive layer: The surface of the photosensitive layer was rubbed 10 times with gauze impregnated with ethanol and 1,1,1-trichloroethane (Dow Chemical Co., Chlorothene), and the degree of stain of the gauze was evaluated according to the following criteria. Excellent: Gauze not stained Good : Gauze somewhat stained Poor : Gauze severely stained The wear resistance of the photosensitive layer was evaluated in terms of pencil hardness according to the following criteria. Excellent: ≧ 2H Good : H Poor : ≦ HB Surface smoothness Print background stain Solvent resistance Wear resistance Example 1 Good Excellent Excellent Good Example 2 Excellent Excellent Excellent Excellent Example 3 Excellent Excellent Excellent Excellent Comparative Example 1 Fair Fair Excellent Excellent Comparative Example 2 Poor Poor Excellent Excellent It is apparent from Table 1 that the printing plates of Examples 1 - 3, which had high degrees of surface smoothness, produced no print background stain, while the printing plates of Comparative Examples 1 and 2, which had rough surfaces, caused background stains. The sensitivity of the photosensitive layer to laser light and the geometry of pits formed by laser irradiation were evaluated according to the following evaluation schema. Both the sensitivity of the photosensitive layer and the geometry of pits of the printing plates according to Examples 1 - 3 were invariably "excellent". Sensitivity of photosensitive layer: The photosensitive layer was irradiated with a semiconductor laser for a predetermined time and the depth of pits produced was measured to evaluate its sensitivity according to the following criteria. Excellent: > 12 µm Good : 12 - 10 µm Poor : ≦ 8 µm Pit geometry or configuration: The pit was examined with an electron microscope and evaluated according to the following criteria. Excellent: A sharp edge with a rather smooth bottom Good : A slightly obscure edge with a corrugated bottom Poor : An obscure edge with a corrugated bottom Examples 4 to 7 A coating composition was prepared by mix-dispersing 7.7 parts by weight of nitrocellulose RS 1/4 (Daicel Chemical Industries, Ltd., isopropyl alcohol-wetted, nonvolatile 70 % by weight), 24.6 parts by weight of a carbon black-containing paste (Toyo Ink Manufacturing Co., Ltd., Color Paste ANP-C-903 Black, carbon black content 12 % by weight, nitrocellulose content 18 % by weight), 43.3 parts by weight of a polyester resin (Toyobo Co., Ltd., Vylon 20SS, nonvolatile 30 % by weight), 5.4 parts by weight of a cross-linking agent (Dainippon Ink and Chemicals, Inc., melamine resin, Super-Beckamine L-105-60, nonvolatile 60 % by weight), 0.4 part by weight of an acid catalyst (BYK Co., BYK Catalyst 450, nonvolatile about 40 % by weight), the predetermined amount (shown below) of a water repellent (a silicone compound or a fluorine-containing compound) and 18.7 parts by weight of methyl ethyl ketone. The amount of the water repellent is based on 100 parts by weight of nonvolatile matter in the coating composition. Example 4: Polydimethylsiloxane (Toshiba Silicone Co., Ltd., TSF-451-100, 100 cst (25°C)), 0.2 part by weight Example 5: Polydimethylsiloxane (Toshiba Silicone, Co., Ltd., TSF-451-1M, 1,000,000 cst (25°C)), 1.0 part by weight Example 6: Silicone gum (Shin-Etu Silicone Co., Ltd.), 0.2 part by weight Example 7: A fluorine-containing compound (Toagosei Chemical Industry, Co., Ltd., Aron G GF-300, fluorine-containing comb-shaped graft polymer), 2.0 parts by weight Using an applicator, the coating composition was coated in a dry thickness of 20 µm on a biaxially oriented polyethylene terephthalate film (100 µm thick) and dried at 120°C for 3 minutes to obtain a printing plate. Examples 8 to 10 A coating composition was prepared by mix-dispersing 6.4 parts by weight of nitrocellulose RS 1/4 (Daicel Chemical Industries, Ltd., isopropyl alcohol-wetted, nonvolatile 70 % by weight), 29.9 parts by weight of carbon black-containing chips (Taisei Kako Co., Ltd., TC Color Chip 5307, carbon black content 30 % by weight, nitrocellulose content 52 % by weight, plasticizer 18 % by weight), 97.2 parts by weight of a polyester resin (Toyobo Co., Ltd., Vylon 30SS, nonvolatile 30 % by weight), 10.1 parts by weight of a cross-linking agent (Mitsui-Cyanamid Co., Ltd., melamine resin, Cymel 325, nonvolatile 80 % by weight), 0.9 part by weight of an acid catalyst (BYK Co., BYK Catalyst 450, nonvolatile about 40 % by weight), the predetermined amount (see below) of a water repellent (a silicone compound or a fluorine-containing compound), and 101.6 parts by weight of methyl ethyl ketone. Using the coating composition thus obtained, a printing plate was obtained in the same manner as in Example 4. Example 8: A fluorine-containing silicone compound (Toagosei Chemical Industry Co., Ltd., Aron G GF-150, a fluorine-silicone-containing comb-shaped graft polymer), 0.5 part by weight Example 9: A silicon-containing compound (Toagosei Chemical Industry Co., Ltd., Aron G GS-30, a silicon-containing comb-shaped graft polymer), 0.2 part by weight Example 10: Polydimethylsiloxane (Toshiba Silicone Co., Ltd., TSF-451-100), 0.5 part by weight Comparative Example 3 A printing plate was obtained in the same manner as in Example 4 without addition of a water repellent. Comparative Example 4 A printing plate was obtained in the same manner as in Example 7 except that the level of addition of the fluorine-containing water repellent (Toagosei Chemical Industry Co., Ltd., Aron G GF-300) was changed to 0.05 part by weight. The surface wettability indexes of the photosensitive layers produced in Examples 4 to 10 and Comparative Examples 3 and 4 were respectively determined and the print background stains were evaluated by test printing. The results are shown in Table 2. The wettability index was determined using the wetting reagent defined in JIS K6768 "Method of Wettability Test of Polyethylene and Polypropylene Films". The printing test was conducted using a water-based printing ink (Toyo Ink Manufacturing Co., Ltd., Aqua GEME 3 Red) and the results were evaluated according to the same criteria as described above. Wettability index in mN/m (dyn/cm) Print background stain Example 4 32 Excellent Example 5 32 Excellent Example 6 33 Excellent Example 7 36 Excellent Example 8 35 Excellent Example 9 36 Excellent Example 10 Comparative 32 Excellent Example 3 Comparative 38 Poor Example 4 37 Good It is apparent from Table 2 that the wettability indexes of the photosensitive layers of the printing plates obtained in Examples 4 to 10 were not greater than 36 mN/m (dyn/cm) and no background stain was found even when printing was carried out with a water-based printing ink. The sensitivity of the photosensitive layer to laser light and the geometry of pints formed by laser irradiation were evaluated in the same manner as described hereinbefore. The sensitivity of the photosensitive layer and the pit geometry of the printing plates obtained in Examples 4 to 10 were invariably rated "Excellent". Examples 11 to 13 A coating composition was prepared by mix-dispersing 7.7 parts by weight of nitrocellulose RS 1/4 (Daicel Chemical Industries, Ltd.), 24.6 parts by weight of a carbon black-containing paste (Toyo Ink Manufacturing Co., Ltd., Color Paste ANP-C-903 Black), 43.3 parts by weight of a polyester resin (Toyobo Co., Ltd., Vylon 20SS), 5.4 parts by weight of a crosslinking agent (Dainippon Ink and Chemicals, Inc., Super-Beckamine L-105-60), 0.4 part by weight of an acid catalyst (BYK Co., BYK Catalyst 450), and 18.7 parts by weight of methyl ethyl ketone. The coating composition for a sensitive layer was coated in a dry thickness of 20 µm on each of the following 188 µm-thick biaxially stretched polyethylene terephthalate films each carrying an adhesive layer and dried at 120°C for 3 minutes. Example 11: Toyobo Co., Ltd., Espet A4100 Example 12: Toray Industries, Inc., Lumilar U98 Example 13: Teijin Limited, Tetron Film HP7 The sensitivity, pit geometry, solvent resistance (ethanol, chlorothene) and wear resistance (pencil hardness) of the photosensitive layer were evaluated. As in Example 1, satisfactory performance characteristics required for laser-sculptured printing plates were obtained. Furthermore, the density of each polyethylene terephthalate film was determined by the water-submerged replacement method and the impact resistance of the printing plate obtained in each example was determined with an impact tester (Toyo Seiki Seisakusyo Co., Ltd., a puncture tester). The results are shown in Table 3. For practical purposes, an impact strength of not less than 0.981 J (10 kg-cm) is a minimum requirement. Comparative Example 5 A printing plate was obtained in the same manner as in Example 11 except that a biaxially stretched polyethylene terephthalate film (ICI Co., Ltd., Merinex 542, 188 µm thick) was used in lieu of the film used in Example 11. The impact resistance of the printing plate obtained in Comparative Example 5 was evaluated in the same manner as above. The result is shown in Table 3. The printing plate having a practically useful impact strength and the plate not having a practically useful impact strength are indicated by the marks "O" and "X", respectively. Density of film (Water-submerged replacement method) Impact strength in J (kg-cm) Useful strength Example 11 1.380 2.08 (21.2) O Example 12 1.380 1.90 (19.4) O Example 13 1.385 1.30 (13.3) O Comparative Example 5 1.390 0.34 (3.5) X It is apparent from Table 3 that when the density of the support polyethylene terephthalate film exceeds 1.39, the impact resistance is remarkably decreased. For investigating the influence of heat in the course of formation of a photosensitive layer, the film used in Comparative Example 5 was not coated with the coating composition but heat-treated under the conditions shown in Table 4 and subjected to the impact test described above. The results are shown in Table 4. Impact strength in J (kg - cm) Non-treatment 2.39 (24.4) 120°C x 20 min. 2.33 (23.8) 120°C x 30 min. 2.32 (23.6) 120°C x 45 min. 2.32 (23.6) Since the film used in Comparative Example 5 underwent little decrease in impact resistance upon heat treatment as shown in Table 4, the observed decrease in impact resistance of Comparative Example 5 was considered not ascribable to heat but to other factors such as the formulation of the coating composition or a synergistic effect of coating formulation and temperature. Reference Examples 1 to 3 Coating compositions were prepared in the same manner as in Example 1 except that, instead of the polyester resin used in Example 1, an acrylic resin (Rohm & Haas Co., Acryloid B-44, Reference Example 1), a modified polybutadiene resin (Nippon Soda Co., Ltd., BN 1015, Reference Example 2), and a vinyl chloride-vinyl acetate copolymer resin (Denki Kagaku Kogyo Co., Ltd., Denka Vinyl 1000C, Reference Example 3) were respectively used. Using each of these coating compositions, a printing plate was obtained as in Example 1. The sensitivity, pit geometry, solvent resistance and wear resistance (pencil hardness) of the photosensitive layers obtained in Examples 1 to 3 and Reference Examples 1 to 3 were evaluated in the same manner as above. The results are shown in Table 5. Species of resin Sensitivity Pit geometry Solvent resistance Wear resistance Example 1 Polyester Excellent Excellent Excellent Good Example 2 Polyester Excellent Excellent Excellent Excellent Example 3 Polyester Excellent Excellent Excellent Excellent Reference Example 1 Acrylic resin Poor Good Excellent Good Reference Example 2 Modified polybutadien resin Poor Good Good Good Reference Example 3 Vinyl chloride-vinyl acetate copolymer resin Measurement is impossible Poor Good Poor It is apparent from Table 5 that all the printing plates obtained in Examples 1 to 3 were not only sensitive enough to sculptured pits corresponding to laser beams but also were high in solvent resistance and wear resistance, thus being of use as laser-sculptured printing plates.

Claims: Use of a material for a printing plate, wherein said material comprises a support having provided thereon a photosensitive layer containing nitrocellulose, a black coloring agent and a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents, said photosensitive layer having (1) a surface roughness of not greater than Rmax. 2.0 µm or (2) a surface tension equivalent to a wettability index of not greater than 36 mN/m. Use of a material for a printing plate according to claim 1, wherein said nitrocellulose has a nitrogen content of 10 to 14%. Use of a material for a printing plate according to claim 1, wherein said nitrocellulose has a degree of polymerization in the range of 10 to 1500. Use of a material for a printing plate according to claim 1, wherein said nitrocellulose has a solution viscosity, according to Japanese Industrial Standards (JIS) K6703, of 20 to 1/10 second. Use of a material for a printing plate according to claim 1, wherein said nitrocellulose has a nitrogen content of 11 to 12.5%, a degree of polymerization in the range of 10 to 900 and a solution viscosity, according to JIS K6703, of 10 to 1/8 second. Use of a material for a printing plate according to claim 1, wherein said black coloring agent is carbon black. Use of a material for a printing plate according to claim 1, wherein said polyester resin is an oil-free aromatic polyester. Use of a material for a printing plate according to claim 7, wherein said aromatic polyester is a modified polyester which is obtainable by replacing part of at least one of terephthalic acid and ethylene glycol constituting polyethylene terephthalate with a different dicarboxylic acid or diol. Use of a material for a printing plate according to claim 1, wherein said photosensitive layer comprises 5 to 300 parts by weight of said nitrocellulose and 0.5 to 50 parts by weight of said black coloring agent relative to 100 parts by weight of said polyester resin. Use of a material for a printing plate according to claim 1, wherein said photosensitive layer further contains a crosslinking agent. Use of a material for a printing plate according to claim 10, wherein the proportion of said crosslinking agent is 0.1 to 50 parts by weight relative to 100 parts by weight of said polyester resin. Use of a material for a printing plate according to claim 1, wherein said photosensitive layer further contains a water repellent. Use of a material for a printing plate according to claim 12, wherein said water repellent is a silicone compound or a fluorine-containing compound. Use of a material for a printing plate according to claim 1, wherein said photosensitive layer has a surface wettability index of 30 to 36 mN/m. Use of a material for a printing plate according to claim 12, wherein the proportion of said water repellent is 0.1 to 5 parts by weight relative to 100 parts by weight of the photosensitive layer. Use of a material for a printing plate according to claim 1, wherein said support is a biaxially stretched or oriented polyethylene terephthalate film. Use of a material for a printing plate according to claim 16, wherein said polyethylene terephthalate film has a density of less than 1.39. Use of a material for a printing plate according to claim 17, wherein said polyethylene terephthalate film has a density of 1.375 to 1.387. Use of a material for a printing plate according to claim 1 which is used in association with a water-based printing ink, and wherein said material comprises a polyethylene terephthalate film layer having a density of 1.375 to 1.387 having provided thereon a photosensitive layer containing nitrocellulose, carbon black, a linear non-crystalline thermoplastic oil-free aromatic polyester resin which is soluble in organic solvents and a crosslinking agent, said photosensitive layer having (1) a surface roughness of not greater than Rmax. 1.0 µm or (2) a surface tension equivalent to a wettability index of not greater than 36 mN/m. Use of a material for a printing plate according to claim 19, wherein the surface of said photosensitive layer has a wettability index of 32 to 36 mN/m as imparted by a water repellent incorporated into said photosensitive layer and selected from the group consisting of silicone oil, silicone gum and a silicone compound- or fluorine-containing compound-grafted polymer. Use of a material for a printing plate according to claim 20, wherein said water repellent is incorporated in a proportion of 0.2 to 2 parts by weight relative to 100 parts by weight of the photosensitive layer. A printing plate which comprises a support having provided thereon a photosensitive layer containing nitrocellulose, a black coloring agent and a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents, said photosensitive layer having (1) a surface roughness of not greater than Rmax. 2.0 µm or (2) a surface tension equivalent to a wettability index of not greater than 36 mN/m. The printing plate according to claim 22, wherein said support is a flexible transparent support. The printing plate according to claim 22, wherein said surface roughness is not greater than Rmax. 1.5 µm. The printing plate according to claim 22, wherein said photosensitive layer comprises 20 to 200 parts by weight of a nitrocellulose having a nitrogen content of 11.5 to 12.2% and a solution viscosity, according to JIS K6703, of 1 to 1/8 second, 5 to 40 parts by weight of carbon black and 5 to 40 parts by weight of a crosslinking agent relative to 100 parts by weight of said polyester resin, and said support is a transparent plastic film. The printing plate according to claim 23, wherein said flexible transparent support comprises a biaxially stretched or oriented polyethylene terephthalate film having a density of 1.375 to 1.387, and wherein said photosensitive layer further contains a crosslinking agent and can be sculptured with laser light. A method for producing a printing plate as defined in claim 22, which comprises coating a coating composition containing nitrocellulose, a black coloring agent and a linear non-crystalline thermoplastic polyester resin which is soluble in organic solvents on a biaxially stretched or oriented polyethylene terephthalate film having a density of less than 1.39 to provide a photosensitive layer which can be sculptured with laser light. The method for producing a printing plate according to claim 27, wherein said coating composition further contains a water repellent.

Inventors: FUNAKI YOSHINORI, NOTSU KAZUO, FUNAKI, YOSHINORI, NOTSU, KAZUO
Applicants: DAICEL CHEM, DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1994
Main classification: B41N1
IPC codes: B41N1, B41C1, G03F7